Influence of neck muscles on mouth pressure response to cervical magnetic stimulation

Sex differences in diaphragmatic fatigue: Effects of hypoxia during inspiratory loading

KEY POINTS

Under normoxic conditions, both healthy female and male diaphragms fatigue at a similar degree when matched for absolute diaphragmatic work during inspiratory loading. We investigated whether similarities in diaphragm fatigability persist under acute hypoxic conditions. We found that, in acute hypoxia, fatigue of the diaphragm is greater in women compared to men, whereas the magnitude of fatigue in normoxia did not differ between sexes. When matched for maximal diaphragm strength, women and men had a similar pressor response to work-matched inspiratory loading, independent of oxygen availability.

ABSTRACT

In normoxia, women and men display a comparable magnitude of diaphragmatic fatigue (DF) after work-matched inspiratory loading. Whether these sex similarities are maintained under acute hypoxic conditions is unknown. We investigated the influence of acute hypoxia during work-matched inspiratory pressure-threshold loading (PTL) on DF in healthy women (n = 8) and men (n = 8). Two 5 min isocapnic PTL tasks targeting a transdiaphragmatic pressure (P_di ) of 92 cmH₂ O in normoxia and hypoxia (8% O₂ ) were performed on separate days (≥48 h). DF was quantified by twitch P_di (P_di,tw ) via cervical magnetic stimulation post-PTL. Women and men had similar maximal P_di (P_di,max ; women: 171 ± 16, men: 178 ± 20 cmH₂ O) and relative target workload (women: 54 ± 5%, men: 53 ± 6% P_di,max ). The absolute cumulative diaphragmatic work did not differ between sexes in normoxia (women: 12,653 ± 1796 cmH₂ O s^-1 , men: 13,717 ± 1231 cmH₂ O s^-1 ; P = 0.202) or hypoxia (women: 11,624 ± 1860 cmH₂ O s^-1 , men: 12 722 ± 1502 cmH₂ O s^-1 ; P = 0.189). In normoxia, the magnitude of reduction in P_di,tw post-PTL was similar between sexes (women: -21.1 ± 8.4%, men: -22.5 ± 4.9 %; P = 0.193); however, a higher degree of DF was observed in women compared to men following PTL in acute hypoxia (women: -27.6 ± 7.7%, men: -23.4 ± 9.6%, P = 0.019). We conclude that the female diaphragm is more susceptible to fatigue after inspiratory loading under acute hypoxic conditions. This finding may be related to sex differences in diaphragm muscle metabolism, such as fibre type composition, contractile properties, substrate utilisation and blood perfusion.

Ultrafast ultrasound coupled with cervical magnetic stimulation for non-invasive and non-volitional assessment of diaphragm contractility

KEY POINTS

Twitch transdiaphragmatic pressure elicited by cervical magnetic stimulation of the phrenic nerves is a fully non-volitional method for assessing diaphragm contractility in humans, yet it requires invasive procedures such as oesophageal and gastric catheter balloons.  Ultrafast ultrasound enables a very high frame rate allowing the capture of transient events, such as muscle contraction elicited by nerve stimulation (twitch). Whether indices derived from ultrafast ultrasound can be used as an alternative to the invasive measurement of twitch transdiaphragmatic pressure is unknown.  Our findings demonstrate that maximal diaphragm tissue velocity assessed using ultrafast ultrasound following cervical magnetic stimulation is reliable, sensitive to change in cervical magnetic stimulation intensity, and correlates to twitch transdiaphragmatic pressure.  This approach provides a novel fully non-invasive and non-volitional tool for the assessment of diaphragm contractility in humans.

ABSTRACT

Measuring twitch transdiaphragmatic pressure (P_di,tw ) elicited by cervical magnetic stimulation (CMS) is considered as a reference method for the standardized evaluation of diaphragm function. Yet, the measurement of P_di requires invasive oesophageal and gastric catheter-balloons. Ultrafast ultrasound is a non-invasive imaging technique enabling frame rates high enough to capture transient events such as evoked muscle contractions. This study investigated relationships between indices derived from ultrafast ultrasound and P_di,tw , and how these indices might be used to estimate P_di,tw . CMS was performed in 13 healthy volunteers from 30% to 100% of maximal stimulator intensity in units of 10% in a randomized order. P_di,tw was measured and the right hemidiaphragm was imaged using a custom ultrafast ultrasound sequence with 1 kHz framerate. Maximal diaphragm axial velocity (V_di ,_max ) and diaphragm thickening fraction (TF_di,tw ) were computed. Intra-session reliability was assessed. Repeated-measures correlation (R) and Spearman correlation coefficients (ρ) were used to assess relationships between variables. Intra-session reliability was strong for P_di,tw and V_di,max and moderate for TF_di,tw . V_di,max correlated with P_di,tw in all subjects (0.64 < ρ < 1.00, R = 0.75; all P < 0.05). TF_di,tw correlated with P_di,tw in eight subjects only (0.85 < ρ < 0.93, R = 0.69; all P < 0.05). Coupling ultrafast ultrasound and CMS shows promise for the non-invasive and fully non-volitional assessment of diaphragm contractility. This approach opens up the prospect of both diagnosis and follow-up of diaphragm contractility in clinical populations.

Human diaphragm atrophy in amyotrophic lateral sclerosis is not predicted by routine respiratory measures

Amyotrophic lateral sclerosis (ALS) patients show progressive respiratory muscle weakness leading to death from respiratory failure. However, there are no data on diaphragm histological changes in ALS patients and how they correlate with routine respiratory measurements.

We collected 39 diaphragm biopsies concomitantly with laparoscopic insertion of intradiaphragmatic electrodes during a randomised controlled trial evaluating early diaphragm pacing in ALS (https://clinicaltrials.gov; NCT01583088). Myofibre type, size and distribution were evaluated by immunofluorescence microscopy and correlated with spirometry, respiratory muscle strength and phrenic nerve conduction parameters. The relationship between these variables and diaphragm atrophy was assessed using multivariate regression models.

All patients exhibited significant slow- and fast-twitch diaphragmatic atrophy. Vital capacity (VC), maximal inspiratory pressure, sniff nasal inspiratory pressure (SNIP) and twitch transdiaphragmatic pressure did not correlate with the severity of diaphragm atrophy. Inspiratory capacity (IC) correlated modestly with slow-twitch myofibre atrophy. Supine fall in VC correlated weakly with fast-twitch myofibre atrophy. Multivariate analysis showed that IC, SNIP and functional residual capacity were independent predictors of slow-twitch diaphragmatic atrophy, but not fast-twitch atrophy.

Routine respiratory tests are poor predictors of diaphragm structural changes. Improved detection of diaphragm atrophy is essential for clinical practice and for management of trials specifically targeting diaphragm muscle function.

Relieving dyspnoea by non-invasive ventilation decreases pain thresholds in amyotrophic lateral sclerosis

BACKGROUND

Dyspnoea is a threatening sensation of respiratory discomfort that presents many similarities with pain. Experimental dyspnoea in healthy subjects induces analgesia. This 'dyspnoea-pain counter-irritation' could, in reverse, imply that relieving dyspnoea in patients with chronic respiratory diseases would lower their pain thresholds.

METHODS

We first determined pressure pain thresholds in 25 healthy volunteers (22-31 years; 13 men; handheld algometer), during unloaded breathing (BASELINE) and during inspiratory threshold loading (ITL). Two levels of loading were used, adjusted to induce dyspnoea self-rated at 60% or 80% of a 10 cm visual analogue scale (ITL6 and ITL8). 18 patients with chronic respiratory failure due to amyotrophic lateral sclerosis (ALS) were then studied during unassisted breathing and after 30 and 60 min of non-invasive ventilation-NIV30 and NIV60-(same dyspnoea evaluation).

RESULTS

In healthy volunteers, pressure pain thresholds increased significantly in the deltoid during ITL6 (p<0.05) and ITL8 (p<0.05) and in the trapezius during ITL8 (p<0.05), validating the use of pressure pain thresholds to study dyspnoea-pain counter-irritation. In patients with ALS, the pressure pain thresholds measured in the deltoid during unassisted breathing decreased by a median of 24.5%-33.0% of baseline during NIV30 and NIV60 (p<0.05).

CONCLUSION

Relieving dyspnoea by NIV in patients with ALS having respiratory failure is associated with decreased pressure pain thresholds. Clinical implications have yet to be determined, but this observation suggests that patients with ALS could become more susceptible to pain after the institution of NIV, hence the need for reinforced attention towards potentially painful diagnostic and therapeutic interventions.

Cortical drive to breathe in amyotrophic lateral sclerosis: a dyspnoea-worsening defence?

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease causing diaphragm weakness that can be partially compensated by inspiratory neck muscle recruitment. This disappears during sleep, which is compatible with a cortical contribution to the drive to breathe. We hypothesised that ALS patients with respiratory failure exhibit respiratory-related cortical activity, relieved by noninvasive ventilation (NIV) and related to dyspnoea.

We studied 14 ALS patients with respiratory failure. Electroencephalographic recordings (EEGs) and electromyographic recordings of inspiratory neck muscles were performed during spontaneous breathing and NIV. Dyspnoea was evaluated using the Multidimensional Dyspnea Profile.

Eight patients exhibited slow EEG negativities preceding inspiration (pre-inspiratory potentials) during spontaneous breathing. Pre-inspiratory potentials were attenuated during NIV (p=0.04). Patients without pre-inspiratory potentials presented more advanced forms of ALS and more severe respiratory impairment, but less severe dyspnoea. Patients with pre-inspiratory potentials had stronger inspiratory neck muscle activation and more severe dyspnoea during spontaneous breathing.

ALS-related diaphragm weakness can engage cortical resources to augment the neural drive to breathe. This might reflect a compensatory mechanism, with the intensity of dyspnoea a negative consequence. Disease progression and the corresponding neural loss could abolish this phenomenon. A putative cognitive cost should be investigated.

Use of a two-way non-rebreathing valve to simplify the measurement of twitch mouth pressure using an inspiratory pressure trigger and the establishment of an optimal trigger threshold for healthy subjects and COPD patients

OBJECTIVE

Controlled twitch mouth pressure (Tw Pmo) via the use of a two-way non-rebreathing valve is a new method to assess diaphragm contractility. The optimal trigger threshold was confirmed.

DESIGN

We sought to determine the optimal trigger threshold for 17 healthy subjects (29±4 years) and 17 COPD patients (64±10 years). The Tw Pmo, twitch oesophageal pressure (Tw Pes) and twitch transdiaphragmatic pressure (Tw Pdi) in response to phrenic nerve stimulation were measured using an inspiratory pressure trigger at -1, -2, -3, -4, -5 and -6 cmH2O.

RESULTS

The lung volume did not change during triggering at different trigger thresholds using a two-way non-rebreathing valve. The highest correlation between Tw Pmo and Tw Pes in healthy subjects and COPD patients occurred for a -2 cmH2O trigger threshold (r=0.939 and r=0.869, P<0.0001). The narrowest limits of agreement for Tw Pmo and Tw Pes both occurred at -2 cmH2O in healthy subjects, with a bias (range) of -0.4 cmH2O (-1.85 to 1.41), and in COPD patients, with a bias (range) of 0.1 6cmH2O (-1.36-1.67).

CONCLUSIONS

We conclude that the measurement of Tw Pmo using a two-way non-rebreathing valve is of clinical value to investigate the suspected diaphragm contractility. The highest trigger threshold for clinical applications was -2 cmH2O.

Noninvasive ventilation reduces energy expenditure in amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) leads to chronic respiratory failure. Diaphragmatic dysfunction, a major driver of dyspnea and mortality, is associated with a shift of the burden of ventilation to extradiaphragmatic inspiratory muscles, including neck muscles. Besides, energy expenditure is often abnormally high in ALS, and this is associated with a negative prognostic value. We hypothesized that noninvasive ventilation (NIV) would relieve inspiratory neck muscles and reduce resting energy expenditure (REE).

METHODS

Using indirect calorimetry, we measured REE during spontaneous breathing (REESB) and NIV (REENIV) in 16 ALS patients with diaphragmatic dysfunction, during the first 3 months of NIV. Measured values were compared with predicted REE (REEpred)(Harris-Benedict equation).

RESULTS

NIV abolished inspiratory neck muscle activity. Even though our patients were not hypermetabolic, on the contrary, with a REESB that was lower than REEpred (average 11%), NIV did reduce energy expenditure. Indeed, median REENIV, in this population with a mean body mass index of 21.4 kg.m-2, was 1149 kcal/24 h [interquartile 970-1309], lower than REESB (1197 kcal/24 h, 1054-1402; mean difference 7%; p = 0.03, Wilcoxon). REESB and REENIV were correlated with forced vital capacity and maximal inspiratory pressure.

CONCLUSIONS

NIV can reduce energy expenditure in ALS patients probably by alleviating the ventilatory burden imposed on inspiratory neck muscles to compensate diaphragm weakness. It remains to be elucidated whether or not, in which population, and to what extent, NIV can be beneficial in ALS through the corresponding reduction in energy expenditure.

Optimal arrangement of magnetic coils for functional magnetic stimulation of the inspiratory muscles in dogs

In an attempt to maximize inspiratory pressure and volume, the optimal position of a single or of dual magnetic coils during functional magnetic stimulation (FMS) of the inspiratory muscles was evaluated in twenty-three dogs. Unilateral phrenic magnetic stimulation (UPMS) or bilateral phrenic magnetic stimulation (BPMS), posterior cervical magnetic stimulation (PCMS), anterior cervical magnetic stimulation (ACMS) as well as a combination of PCMS and ACMS were performed. Trans-diaphragmatic pressure (Pdi), flow, and lung volume changes with an open airway were measured. Transdiaphragmatic pressure was also measured with an occluded airway. Changes in inspiratory parameters during FMS were compared with 1) electrical stimulation of surgically exposed bilateral phrenic nerves (BPES) and 2) ventral root electrical stimulation at C5-C7 (VRES C5-C7). Relative to the Pdi generated by BPES of 36.3 +/- 4.5 cm H2O (Mean +/- SEM), occluded Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and a combined PCMS + ACMS were 51.7%, 61.5%, 22.4%, 100.3%, and 104.5% of the maximal Pdi, respectively. Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and combined ACMS + PCMS were 38.0%, 45.2%, 16.5%, 73.8%, and 76.8%, respectively, of the Pdi induced by VRES (C5-C7) (48.0 +/- 3.9 cm H2O). The maximal Pdi(s) generated during ACMS and combined PCMS + ACMS were higher than the maximal Pdi(s) generated during UPMS, BPMS, or PCMS (p < 0.05). ACMS alone induced 129.8% of the inspiratory flow (73.0 +/- 9.4 L/ min) and 77.5% of the volume (626 +/- 556 ml) induced by BPES. ACMS and combined PCMS + ACMS produce a greater inspiratory pressure than UPMS, BPMS or PCMS. ACMS can be used to generate sufficient inspiratory pressure, flow, and volume for activation of the inspiratory muscles.

Disorders of the respiratory muscles

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Is weaning failure caused by low-frequency fatigue of the diaphragm?

Because patients who fail a trial of weaning from mechanical ventilation experience a marked increase in respiratory load, we hypothesized that these patients develop diaphragmatic fatigue. Accordingly, we measured twitch transdiaphragmatic pressure using phrenic nerve stimulation in 11 weaning failure and 8 weaning success patients. Measurements were made before and 30 minutes after spontaneous breathing trials that lasted up to 60 minutes. Twitch transdiaphragmatic pressure was 8.9 +/- 2.2 cm H2O before the trials and 9.4 +/- 2.4 cm H2O after their completion in the weaning failure patients (p = 0.17); the corresponding values in the weaning success patients were 10.3 +/- 1.5 and 11.2 +/- 1.8 cm H2O (p = 0.18). Despite greater load (p = 0.04) and diaphragmatic effort (p = 0.01), the weaning failure patients did not develop low-frequency fatigue probably because of greater recruitment of rib cage and expiratory muscles (p = 0.004) and because clinical signs of distress mandating the reinstitution of mechanical ventilation arose before the development of fatigue. Twitch pressure revealed considerable diaphragmatic weakness in many weaning failure patients. In conclusion, in contrast to our hypothesis, weaning failure was not accompanied by low-frequency fatigue of the diaphragm, although many weaning failure patients displayed diaphragmatic weakness.

Can diaphragmatic contractility be assessed by airway twitch pressure in mechanically ventilated patients?

BACKGROUND

In critically ill patients inspiratory muscle function may be assessed by measurements of maximal inspiratory airway pressure and the response of twitch transdiaphragmatic pressure (Pdi tw) to bilateral phrenic nerve stimulation. The first is limited by its total dependence on patient cooperation. Although the second approach is independent of patient volition, it is impractical because it requires oesophageal and gastric balloons. Because airway pressure is easily and non-invasively recorded in patients with artificial airways, we hypothesised that twitch airway pressure (Paw tw) reliably predicts Pdi tw and twitch oesophageal pressure (Poes tw) in mechanically ventilated patients.

METHODS

Thirteen mechanically ventilated patients recovering from an episode of acute respiratory failure received phrenic nerve stimulation at end exhalation. The rapid occlusion technique was used to record respiratory system mechanics.

RESULTS

Stimulations were well tolerated. Mean (SE) Paw tw at end exhalation was -8.2 (1.2) cm H(2)O and Poes tw and Pdi tw were -7.3 (1.1) and 10.4 (1.8) cm H(2)O, respectively. Stimulations produced a good correlation between Paw tw and Pdi tw (p<0.001), although the limits of agreement were wide. The results were similar for Poes tw. No relationship was found between the Paw tw/Poes tw ratio and respiratory system compliance or airway resistance. Paw tw reproducibility was excellent (mean coefficient of variation 6%, range 3-9%).

CONCLUSIONS

Despite a good correlation between Paw tw and Poes tw, Paw tw did not reliably predict Poes tw or Pdi tw in mechanically ventilated patients. Nevertheless, the excellent reproducibility of Paw tw suggests that it may be a useful means of monitoring inspiratory muscle contractility in the routine care of mechanically ventilated patients.

Validation of improved recording site to measure phrenic conduction from surface electrodes in humans

Phrenic nerve stimulation, electrical (ES) or from cervical magnetic stimulation (CMS), allows one to assess the diaphragm contractile properties and the conduction time of the phrenic nerve (PNCT) through recording of an electromyographic response, traditionally by using surface electrodes. Because of the coactivation of extradiaphragmatic muscles, signal contamination can jeopardize the determination of surface PNCTs. To address this, we compared PNCTs with ES and CMS from surface and needle diaphragm electrodes in five subjects (10 phrenic nerves). At a modified recording site, lower and more anterior than usual (lowest accessible intercostal space, costochondral junction) with electrodes 2 cm apart, surface and needle PNCTs were similar (CMS: 6.0 +/- 0.25 ms surface vs. 6.2 +/- 0.13 ms needle, not significant). Electrodes recording the activity of the most likely sources of signal contamination, i.e., the serratus anterior and pectoralis major, showed distinct responses from that of the diaphragm, their earlier occurrence strongly arguing against contamination. With ES and CMS, apparently uncontaminated signals could be consistently recorded from surface electrodes.

Whistle mouth pressure as test of expiratory muscle strength

Expiratory muscle strength is a determinant of cough function. Mouth pressures during a maximal static expiratory effort (PE,max) are dependent on patient motivation and technique and low values are therefore difficult to interpret. This study hypothesized that a short, sharp and maximal expiration through a narrow aperture, a "whistle", might provide a complementary test of expiratory muscle strength. To obtain a maximal whistle, subjects (27 healthy volunteers and 10 patients with amyotrophic lateral sclerosis) were asked to perform a short, sharp blow as hard as possible, from total lung capacity, through a reversed paediatric inhaler whistle, connected to a flange-type mouthpiece. In both healthy subjects and patients, whistle mouth pressure (Pmo,W) was closely related to the pressure measured in the oesophagus and stomach during the same manoeuvre. In healthy subjects, Pmo,W and PE,max correlated with wide limits of agreement, although Pmo,W values were significantly higher than PE,max (131+/-31 cmH2O versus 101+/-27 cmH2O, p<0.0001). In patients, it was also found that Pmo,w and PE,max values were strongly related (r=0.937, p<0.0001). In healthy subjects, the intraclass correlation coefficient and the variation coefficient for Pmo,W repeated measurements were respectively 0.88 and 7.0%. However Pmo,W and PE,max were always smaller than the gastric pressure generated by a maximal cough. It is concluded that mouth whistle pressure, a noninvasive, reproducible and simple test, provides a reliable measure of expiratory muscle strength in healthy subjects that is acceptable to patients and can be used in a complementary fashion to maximal static expiratory effort.

Bilateral phrenic paralysis in a patient with systemic lupus erythematosus

Respiratory manifestations of systemic lupus erythematosus (SLE) are frequent. They include respiratory muscle abnormalities, which have been implicated in the pathogenesis of the "shrinking lung syndrome" (SLS). We report the case of a patient with this syndrome, in whom diaphragmatic paralysis due to demyelinating phrenic lesions was diagnosed at the same time as SLE. Follow-up studies showed a favorable clinical and diaphragmatic outcome with corticosteroid therapy, but little change in spirometry. It is concluded that severe diaphragm palsy is possibly due to phrenic nerve lesions in SLE, and that the link between diaphragm dysfunction and the SLS is probably not a straightforward one.

Sleep disorders and diaphragmatic function in patients with amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), the progressive loss of upper and lower motor neurons leads to respiratory failure, often with predominant diaphragm dysfunction, and death. Because the diaphragm is the only active inspiratory muscle during rapid eye movement (REM) sleep, there is a high theoretical risk of respiratory disorders during REM sleep in patients with ALS. To assess this hypothesis, we studied sleep characteristics (polysomnography) in 21 patients with ALS, stratified according to the presence or absence of diaphragmatic dysfunction. Diaphragmatic dysfunction was defined as an absent or delayed diaphragm response to cervical or cortical magnetic stimulation, abdominal paradox, or respiratory pulse (Group 1, 13 patients). These patients did not differ in age, clinical course, or form (bulbar or spinal) from the eight others, who did not have diaphragmatic dysfunction (Group 2). REM sleep was reduced in Group 1 (7 +/- 7% of total sleep time; mean +/- SD) and normal in Group 2 (18 +/- 6%, p = 0.004). Apneas or hypopneas were rare in both groups. In Group 1, REM sleep was absent or minimal (less than 3 min) in five patients. An unusual and remarkable preservation of phasic inspiratory sternomastoid activation during REM was associated with longer REM sleep duration in six of the other patients with diaphragmatic dysfunction. Median survival time was dramatically shorter (217 d) in Group 1 than in Group 2 (619 d, p = 0.015).

Neuromuscular disease and hypoventilation

Alveolar hypoventilation associated with neuromuscular disease can occur in acute and chronic forms. In the acute form, progressive weakness of respiratory muscles leads to rapid reduction in vital capacity followed by respiratory failure with hypoxemia and hypercarbia. Symptoms are those of acute respiratory failure, including dyspnea, tachypnea, and tachycardia. In the chronic form, impairment of the respiratory muscles affects mechanical properties of the lungs and chest wall, decreases the ability to clear secretions, and eventually may alter the function of the central respiratory centers. Symptoms include orthopnea, fatigue, disturbed sleep, and hypersomnolence. Treatment and outcome of the disease's chronic form are dependent on the underlying clinical cause of the alveolar hypoventilation. For chronic but stable diseases such as old polio, quadriplegia, or kyposcoliosis, mechanical support of minute ventilation can reverse symptoms. For chronic and progressive disease such as muscular dystrophy and amyotrophic lateral sclerosis, mechanical support of minute ventilation provides only symptomatic relief and is usually associated with deterioration to the point of complete ventilator dependency for survival. For the chronic progressive forms of alveolar hypoventilation, there is currently a need for quality randomized controlled clinical trials to define physiologic indicators and appropriate timing for mechanical support of minute ventilation.

Can diaphragmatic contractility be assessed by twitch airway pressures in patients with chronic obstructive pulmonary disease?

In healthy subjects and in patients without lung diseases, twitch airway pressure (Paw(tw)) responses to phrenic nerve stimulation can be used to predict twitch esophageal pressure (Pes(tw)) and twitch transdiaphragmatic pressure (Pdi(tw)), thus overcoming the need for placement of esophageal and gastric balloons. The aim of this study was to determine whether measurements of Paw(tw) combined with simple maneuvers could be used to predict Pes(tw), and possibly Pdi(tw), in patients with severe chronic obstructive pulmonary disease (COPD) (n = 12). Stimulations delivered at relaxed FRC produced a correlation coefficient (r) between Paw(tw) and Pes(tw) of 0.44 (p < 0.001) and of 0.62 (p < 0.001) during stimulations while patients performed a gentle exhalation from FRC. Stimulations performed during a gentle inhalation produced a good correlation between Paw(tw) and Pes(tw) (r = 0.92, p < 0.001); however, the limits of agreement between Paw(tw) and Pes(tw) were wide. Correlations between Paw(tw) and Pdi(tw) during the three experimental conditions were weak. In conclusion, during a gentle inspiratory effort in patients with severe COPD the correlation between Paw(tw) and Pdi(tw) was weak, whereas the correlation between Paw(tw) and Pes(tw) was good, but it was not sufficient to allow the prediction of Pes(tw) from Paw(tw) in all patients.

Effect of brachial plexus co-activation on phrenic nerve conduction time

BACKGROUND

Diaphragm function can be assessed by electromyography of the diaphragm during electrical phrenic nerve stimulation (ES). Whether phrenic nerve conduction time (PNCT) and diaphragm electrical activity can be reliably measured from chest wall electrodes with ES is uncertain.

METHODS

The diaphragm compound muscle action potential (CMAP) was recorded using an oesophageal electrode and lower chest wall electrodes during ES in six normal subjects. Two patients with bilateral diaphragm paralysis were also studied. Stimulations were deliberately given in a manner designed to avoid or incur co-activation of the brachial plexus.

RESULTS

For the oesophageal electrode the PNCT was similar with both stimulation techniques with mean (SE) values of 7.1 (0.2) and 6.8 (0.2) ms, respectively (pooled left and right values). However, for surface electrodes the PNCT was substantially shorter when the brachial plexus was activated (4.4 (0.1) ms) than when it was not (7.4 (0.2) ms) (mean difference 3.0 ms, 95% CI 2.7 to 3.4, p<0.0001). A small short latency CMAP was recorded from the lower chest wall electrodes during stimulation of the brachial plexus alone.

CONCLUSIONS

The results of this study show that lower chest wall electrodes only accurately measure PNCT when care is taken to avoid stimulating the brachial plexus. A false positive CMAP response to phrenic stimulation could be caused by inadvertent stimulation of the brachial plexus. This finding may further explain why the diaphragm CMAP recorded from chest wall electrodes can be unreliable with cervical magnetic stimulation during which brachial plexus activation occurs.

Cervical magnetic stimulation as a method to discriminate between diaphragm and rib cage muscle fatigue

Inspiratory muscle fatigue can probably determine hypercapnic respiratory failure. Diaphragm fatigue is detected by electrical phrenic stimulation (ELS), but there is no simple tool to assess rib cage muscle (RCM) fatigue. Cervical magnetic stimulation (CMS) costimulates the phrenic nerves and RCM. We reasoned that changes in transdiaphragmatic pressure twitch (Pdi,tw) with CMS and ELS should be different after selective diaphragm vs. RCM fatigue. Five volunteers performed inspiratory resistive tasks while voluntarily uncoupling diaphragm and RCM. Baseline Pdi,twELS and Pdi,twCMS were 28.57 +/- 1.68 and 32.83 +/- 2.92 cmH2O. After selective diaphragm loading, Pdi,twELS and Pdi,twCMS were reduced by 39 and 26%, with comparable decreases in gastric pressure twitch (Pga,tw). Esophageal pressure twitch (Pes,tw) was better preserved with CMS. Therefore Pes,tw/Pga,tw was lower with ELS than CMS (-1.24 +/- 0.16 vs. -1.73 +/- 0.11, P = 0.05). After selective RCM loading, there was no diaphragm fatigue, but Pes,twCMS was significantly reduced (-30%). These findings support the role of rib cage stiffening by CMS-related RCM contraction in the ELS-CMS differences and suggest that CMS can be used to assess RCM fatigue.